Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/04—Continuous casting of metals, i.e. casting in indefinite lengths into open-ended moulds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/10—Supplying or treating molten metal
  • B22D11/11—Treating the molten metal
  • B22D11/114—Treating the molten metal by using agitating or vibrating means
  • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields

Definitions

• the invention relates to a continuous casting machine for the continuous casting of molten metal in particular molten steel into a cast product, comprising a mould in which the molten metal is poured through an exit port of pouring means, forming a bath of molten metal, and in which at least part of the metal is solidified, to a mould suitable for such continuous casting machine and to a method for the operation thereof.
• a continuous casting machine as referred to in this specification may be any of the known continuous casting machines such as a conventional casting machine for casting slabs having a thickness of about 250 mm or a thin slab casting machine for casting slabs having a thickness of about 150 mm or less e.g. in the range 50 - 100 mm.
• molten metal is poured from a tundish into the mould through a submerged entry nozzle as pouring means connected to the tundish and reaching into the mould.
• the centre line of the nozzle generally corresponds with the centre line of the mould.
• a continuous casting machine of the referred type is well known in the art e.g. from WO 95/20445.
• a mould and a nozzle suitable for such a continuous casting machine are known from WO 95/20443.
• a further embodiment of a nozzle is known from EP 0 685 282.
• the molten metal after entering the mould forms recirculations of unequal magnitude and shape.
• two recirculations develop in the vertical plane on either side of the nozzle: a smaller one and a large one.
• the recirculations extend to the meniscus and cause a disturbance thereof, which disturbance is different for each of the two recircula-tions .
• the heat transfer by the circulating molten metal to the casting powder, floating on the surface of the molten bath, and therefore the temperature of the casting powder is dif- ferent for the two recirculations. Consequently the effect of the casting powder on the heat transfer of the molten metal to the chilled walls of the mould is not uniform.
• the recirculations may also lead to entrapment of casting powder and other inclusions into the bath of molten metal.
• the resulting effect, apart from surface and bulk defects, is that the cast thin slab is not uniform in temperature and because of the unpredictability of the position of each of the recirculations, the temperature distribution is not predictable ultimately resulting in a non-uniform thickness, or in other words shape-defects, of the cast slab.
• a direction in which in the prior art a solution was sought was the shape of the nozzle and of the exit ports thereof. Numerous proposals for the shape of the exit port, its angle relation to the longitudinal axis of the nozzle and the shape of the bottom of the nozzle were made. In thin slabs this necessitated a funnel shape of the mould.
• An object of the invention is to provide a continuous casting machine with which these problems can be obviated or at least largely reduced and with which also other advantages can be obtained.
• a continuous casting machine that is characterized in that, it is provided with control means for controlling and/or steering the flow of molten metal and operative on the molten metal after entering the mould such that the flow pattern of the molten metal in the mould is basically symmetrical with respect to at least one plane of symmetry of the mould.
• the invention starts from the idea that the desired symmetry and stability are very difficult to achieve because the flow of molten metal and its behaviour in the mould depends on many factors such as temperature and chemical composition of the molten metal, irregularities in the shape of the nozzle and changes therein during its lifetime because of wear and clogging, temperature gradients over the cooled walls of the mould, deviation in the shape of the mould. All these factors influence the flow in the mould and because each of these factors is difficult to predict or control, the flow is difficult to predict or control by selecting the shape of the nozzle.
• control means are provided that cause a symmetrical flow or in other words, cause symmetrical and basically identical recirculations in the mould and eventually in the not solidified portion of the cast slab, by controlling and or steering the flow of the molten metal after it has entered the mould through the nozzle.
• unsymmetrical or unstable behaviour of the flow of molten metal is not primarily sought to be corrected by selecting the shape of the nozzle and its exit port or ports but by influencing the resulting flow of the metal in the mould and eventually in the non-solidified portion of the cast slab.
• control means comprise at least one magnetic brake apparatus preferably one electro magnetic brake apparatus.
• Electro magnetic brakes for performing a stirring or braking action on a molten metal flow are well known in the art and have proven to be a reliable piece of equipment.
• the electromagnetic brakes is used for stirring a bath of molten metal.
• Electromagnetic stirrers are used for stirring the liquid metal between solidified dendritic solid crystals to remelt these crystals locally along the long axes and to form equiaxed shaped solidified crystals.
• the velocity of the liquid metal leaving the exit port of the entry nozzle is 10 to 100 times the casting speed.
• Electromagnetic brakes are used to brake this high velocity flow of liquid metal entering the mould to prevent deep penetration of the inflowing liquid metal, thereby preventing deep penetration of unwanted inclusions.
• the beneficial effects of electro- magnetic stirrers or brakes .the flow of liquid metal in the mould is not acceptable in view of instability and asymmetry. These unwanted phenomena are not prevented with the electromagnetic brakes and stirrers due to the practical operation.
• static magnetic brakes are suitable it is preferred to use electromagnetic brakes because of the obtainable higher magnetic induction and the simplicity of controlling the magnetic induction by changing the current in the induction coils, in particular DC- or low frequency operated electromagnetic brakes.
• control means in this embodiment through the generation of an electromagnetic force field, effectively obstruct a periodic oscillation phenomena of liquid metal and an asymmetric flow in the mould, resulting in a very stable molten bath surface even in a condition of high casting speed of 2,0 m/min or more for conventional continuous casting machines and 4,0 m/min or more for thin slab casters, leading to a very sound and uniform solidified shell of solidified metal in the mould.
• the control means cause the recirculation to be basically equal and stable.
• the productivity of the continuous casting machine in other words the economics, is dependent on the casting speed and can be substantially increased using the invention.
• a very efficient embodiment of the invention is characterized in that the magnetic brake apparatus comprises two sets of magnetic braking poles spaced apart and operative in a direction basically perpendicular to the direction of the flow of molten metal entering the mould through the exit port.
• each set of poles has a main distribution of the magnetic field perpendicular to the flow of molten metal entering the mould.
• control means are positioned symmetrically with respect to the exit port of the pouring means.
• control means operate very efficiently in an embodiment of the invention that is characterized in that the control means extend in a direction basically perpendicular to the direction of a flow of molten metal entering the mould through the exit port.
• control means are operative within a range between 1/8 and 7/8 of the width of the mould.
• This embodiment allows for sufficient flow of molten metal to the meniscus while stabilizing the remaining flow.
• control means comprising separating means for separating the flow of metal entering the mould in at least two subflows and for obstructing flow from one subflow to a second subflow in both parallel and funnel shaped mould.
• the control means in principle divides the main flow of molten metal into two subflows in general of recirculation-shape, of equal magnitude. Unsymmetry means that one recirculation differs in magnitude from the other recirculation, unsymmetry therefore means that molten metal should pass the control means. Since such passage is obstructed by the control means, the recirculations and therefore the flow in the mould are basically equal and stable.
• the separating means comprise at least one set of magnetic poles, more preferably a set of electromagnetic poles. In a very effective embodiment the separating means is a multiplying factor 1,5 up to 10 longer in the direction of casting than in the direction perpendicular thereto, i.e. the width of the mould.
• control means extend mainly perpendicular with respect to the flow of the molten metal.
• control means are operative only over part of the longest side i.e. width of the mould, preferably between 1/8 and 7/8 thereof, each pole resulting in a main distribution of the magnetic field strength perpendicular to the flow of the molten metal entering the mould.
• control means as magnetic brake brakes and equalizes, due to the velocity dependency of the braking action, the main flow while giving a circulating flow the possibility to extend to the meniscus for the desired heat transfer. High velocity and disturbing recirculations occurring at the outer ends of the magnetic brakes pass through the brakes and are efficiently braked and reduced.
• another embodiment of the continuous casting machine according to the invention is characterized in that the continuous casting machine is provided with braking means for lowering the velocity of the molten metal flowing at the meniscus of the bath of molten metal in the mould.
• braking means for lowering the velocity of the molten metal flowing at the meniscus of the bath of molten metal in the mould.
• a still smaller velocity at the meniscus is required, mainly to prevent disturbance of the meniscus and entrapment of particles of casting powder in the molten metal.
• the velocity at the meniscus can be reduced without essentially influencing the equalizing and stabilizing effect of the control means.
• a very efficient, reliable and easy-to-operate braking means is characterized in that, the braking means comprise at least two magnetic brakes preferably two electro magnetic brakes positioned symmetrically with respect to at least one plane of symmetry of the mould and operative on the flow of metal directed to the meniscus of the molten metal.
• the recirculations occurring in the mould are directed upwardly near the short walls of the mould. Placing the braking means at this position, were the velocity is relatively high, a particular efficient braking effect is obtained with magnetic brakes.
• the position of the control means is variable with respect to the mould.
• the control means it is possible to place the control means in an optimum position in dependency of the mould and nozzle used. It is even possible to adapt the position to varying process conditions, while casting.
• the position of the braking means is variable with respect to the mould.
• an optimum position of the braking means in dependency of mould, nozzle and process conditions can be chosen and maintained even when process conditions vary.
• the invention is also embodied in a mould provided with control means according to the invention and the further embodiments thereof and in a mould suitable for operation with such control means.
• the invention is further embodied in a method for casting steel using a continuous casting machine according to the invention and embodiments thereof.
• the method is characterized in that the operation and/or position of the control means and/or brake means is selected in dependence of the temperature of the molten metal in the meniscus area.
• a still further embodiment is characterized in that the operation and/or position of the control means and/or brake means is selected in dependence of the flow characteristics of the nozzle in the mould.
• V ra ⁇ an means the mean measured velocity at the meniscus.
• the figures show the result of experiments conducted in a water model simulating the mould wherein water is used to simulate molten steel. It is known in the art that such modelling gives a very good representation of the actual behaviour of molten steel is a mould.
• the water model has a rectangular cross-section of sizes 1500 mm width and 100 mm thickness in Fig. 1 - 6.
• Fig. 1 shows the flow pattern as occurs in the prior art apparatus.
• the flow is highly unsymmetrical.
• the measured velocities are shown in the following table.
• Fig. 2 shows the flow pattern wherein control means are applied ro the mould, the control means being for example a magnetic brake simulated by a mesh-type restriction.
• the letter A designates the distance between the exit port of the entry nozzle and the control means. Part of the water passes, braked, the control means, part is deflected upwardly and causes the desired heat flow to the surface of the bath. At the end of the control means, small recirculations occur which are effectively braked by the control means.
• Fig. 3 shows the flow pattern obtained with another embodiment of the invention.
• the magnetic brakes comprises two sets of poles spaced apart in a direction basically perpendicular to the direction of the flow of molten metal. The centre position of the flow passes the brake unobstructed. The side portion, which cause the recirculations are braked and equalized leading to a symmetrical and relative low velocity of the recirculations. The measured results are shown in the following table.
• Fig. 4 shows a further embodiment wherein the control means comprise separating means embodied in a vertically placed magnetic brakes as simulated by a mesh-type control means, acting as an obstruction.
• the control means splits the main flow in two subflows. Each subflow forming a recirculation. Once the main flow has been split in two symmetrically operating recirculations, instability and unsymmetry is prevented by the obstruction effect of the control means.
• the splitting effect initiates the recirculations which prevent that the main flow enters deep into the bath and might thereby entail unwanted inclusions deep into the bath where they might be entrapped and included in the solidified metal " such as steel. Entrapped inclusions may lead to serious defects in the final product.
• this embodiment is relative insensitive to the position of the control means relative to the entry nozzle in any direction. Also therefore this embodiment is very effective.
• FIG. 5 shows braking means for lowering the velocity of the flowing water at the meniscus of the bath.
• the velocity at the surface is relative high. Such high velocity may cause a disturbance at the meniscus resulting in entrapment of melting powder particles such as in case of a steel bath.
• the velocity at the surface of the bath can be reduced to safe values without the risk of freezing of the meniscus.
• the measurement results are shown in the following table.
• Fig. 7 shows another embodiment of the invention, in this case applied to a bifurcated nozzle and a funnel shaped mould.
• the casting speed was raised to 8 m/min.
• a magnetic brake simulated by a mesh-type control means is provided.
• Wave heights are equal for the left side and the right side and can be as low as 3 mm.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Engine Equipment That Uses Special Cycles (AREA)
• Moulding By Coating Moulds (AREA)
• Casting Devices For Molds (AREA)
• Encapsulation Of And Coatings For Semiconductor Or Solid State Devices (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)

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• 1996
  • 1996-09-19 EP EP96202615A patent/EP0832704A1/de not_active Withdrawn
• 1997
  • 1997-09-18 DE DE69702268T patent/DE69702268T2/de not_active Expired - Lifetime
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  • 1997-09-18 WO PCT/NL1997/000525 patent/WO1998012008A1/en not_active Ceased
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• 1998
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